We know that, transistor can be used as an amplifier. For an amplifier, two terminals are required to supply the weak signal and two terminals to collect the amplified signal. Thus four terminals are required but a transistor is said to have only three terminals Therefore, one terminal is used common for both input and output.
This gives rise to three different combinations.
1. Common base configuration (CB)
2. Common emitter configuration (CE)
3. Common collector configuration (CC)
1. CB configuration
A simple circuit arrangement of CB configuration for pnp transistor is shown below.
In this configuration, base is used as common to both input and output. It can be noted that the i/p section has an a.c. source Vi along with the d.c. source VEB. The purpose of including VEB is to keep EB junction always forward biased (because if there is no VEB then the EB junction is forward biased only during the +ve half-cycle of the i/p and reverse biased during the –ve half cycle). In CB configuration, IE –i/p current, IC –o/p current.
2. CE configuration
In this configuration the input is connected between the base and emitter while the output is taken between collector and emitter. For this configuration IB is input current and IC is the output current.
3. CC configuration
In this configuration the input is connected between the base and collector while the output is taken between emitter and collector.
Here IB is the input current and IE is the output current.
Comparison between CB, CC and CE configuration
Input reistance (Ri)
Output resistance (Ro)
Phase relationship between input and output
For high frequency applications
Less than unity
For audio frequency applications
Greater than unity
Grater than unity
For impedance matching
Less than unity